What is source-string-number ?

Trying to use #line directive in GLSL, in the manual it says #line can have source-string-number. Don't know exactly what it is
I guess you can assign a number to a shader, but in that case, how can I fetch it in the program ?

Also didn't find anything useful in the documentation, could someone give me a hint on this ?

Trying to use #line directive in GLSL, in the manual it says #line can have source-string-number. Don't know exactly what it is

It's an integer constant containing the (zero-based) index of the current source string in the array of strings passed as the string parameter to glShaderSource(). It's accessible in the program via the __FILE__ macro.